Method of hybrid packaging a lead frame based multi-chip semiconductor device with multiple interconnecting structures

ABSTRACT

A hybrid packaging multi-chip semiconductor device comprises a lead frame unit, a first semiconductor chip, a second semiconductor chip, a first interconnecting structure and a second interconnecting structure, wherein the first semiconductor chip is attached on a first die paddle and the second semiconductor chip is flipped and attached on a third pin and a second die paddle, the first interconnecting structure electrically connecting a first electrode at a front surface of the first semiconductor chip and a third electrode at a back surface of the second semiconductor chip and a second electrode at the front surface of the first semiconductor chip is electrically connected by second interconnecting structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of a commonly owned pending US application entitled “Hybrid Packaged Lead Frame Based Multi-Chip Semiconductor Device with Multiple Semiconductor Chips and Multiple Interconnecting Structures”, by Hamza Yilmaz et al with application Ser. No. 13/913,770, filing date Jun. 10, 2013, is herein incorporated by reference for any and all purposes.

FIELD OF THE INVENTION

The invention generally relates to a power device, particularly relating to a thin power device with improved heat dissipation and a preparation method thereof.

DESCRIPTION OF THE RELATED ART

Traditionally, wire bonding or metal clip is used for interconnection in a power device. For example, as shown in FIGS. 1A to 1C disclosed in U.S. Pat. No. 6,870,254B1, a chip 15 is attached on a lead frame 11, then a metal clip 16 is attached on the chip 15 for connecting the electrode of the chip 15 with a pin 13 located close to the lead frame 11, where one side of the metal clip 16 is bent downward to mechanically and electrically connected to a V-groove 18 of the pin 13. The heat dissipation performance of the single-chip package shown in FIG. 1C is good, but the heat dissipation performance of a stacked multi-chip package such as a DC-DC converter including a high-side MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) and a low-side MOSFET is poor. As shown in FIG. 2, a low-side MOSFET 25 is attached on a paddle 20, and then a metal clip 26 is attached on the top of the low-side MOSFET 25 with one side of the metal clip 26 bent downward to contact to a pin 22 located close to the paddle 20. A high-side MOSFET 27 is attached on the metal clip 26 and another metal clip 28 is attached on the top of the high-side MOSFET 27 with one side of the metal clip 28 bent downward to contact with a pin 21 located close to the paddle 20. Thus, two MOSFETs are electrically connected to the paddle 20 and the pins in a stacking structure as shown in FIG. 2. However, the chip and clip stacking structure in FIG. 2 is so thick and the head dissipation performance is thus poor.

It is within this context that embodiments of the present invention arise.

BRIEF DESCRIPTION OF THE DRAWINGS

As shown in attached drawing, the embodiment of the invention is more sufficiently described. However, the attached drawing is only used for explaining and illustrating rather than limiting the scope of the invention.

FIGS. 1A to 1C illustrate the steps of forming a single-chip power device in the background art.

FIG. 2 illustrates a stacked double-chip power device in the background art.

FIGS. 3A and 3B are schematic structural diagrams illustrating the top and the bottom of a lead frame unit of this invention.

FIGS. 4A and 4B are schematic structural diagrams illustrating the steps of attaching two MOSFETs on the lead frame unit and mounting the metal clips on the MOSFETs.

FIG. 5A is a perspective view of a hybrid multi-chip package structure after attaching two MOSFETs on the lead frame unit and mounting the metal clips on the MOSFETs and FIG. 5B is a cross sectional diagram of the structure of FIG. 5A along the line AA.

FIGS. 6, 7A and 7B shows alternative embodiments with the bridge clip including different side wall structures.

FIG. 8 shows an alternative embodiment with the small clip being replaced by a bonding wire.

FIG. 9 is a schematic diagram showing an alternative multi-ship package compared with that in FIG. 5A.

FIGS. 10A and 10B are schematic diagrams of the top and bottom of the multi-chip package in FIG. 5A.

FIGS. 11A to 11B are schematic diagrams of the top and bottom of the multi-chip package in FIG. 7B.

FIGS. 12A to 12F are cross sectional schematic diagram illustrating a method for forming the high-side MOSFET and low-side MOSFET.

FIGS. 13A to 13C are top views showing the step of covering the front surface of the wafer with a plastic package material and cutting it to separate individual chips.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3A is a schematic diagram of a lead frame unit. Generally, each lead frame strip includes a plurality of such lead frame units; however, only one lead frame unit is described for the sake of simplicity. The lead frame unit includes a first die paddle 111 and a second die paddle 112 separated from each other and the first to the fourth pins 101-104 respectively. The first die paddle 111 and second die paddle 112 are arranged side by side in a same plane. For convenience, the direction of the longer sides of the first or the second die paddle is defined as the longitudinal direction and that of the shorter side is defined as the transverse direction. The first die paddle 111 includes a first transverse edge 111 a and second transverse edge 111 b opposite to each other, and a first longitudinal edge 111 c and a second longitudinal edge 111 d opposite to each other. The second die paddle 112 includes a first transverse edge 112 a and a second transverse edge 112 b opposite to each other and a first longitudinal edge 112 c and second longitudinal edge 112 d opposite to each other. The second longitudinal edge 111 d of the first die paddle 111 is close to and substantially parallel to the first longitudinal edge 112 c of the second die paddle 112. The first pin 101 and the second pin 102 are also arranged side by side collinearly and positioned close to the first longitudinal edge 111 c of the first die paddle 111 and a connecting rib connected to the first longitudinal edge 111 c of the first die paddle 111 is arranged between the first pin 101 and the second pin 102, which is used for a mechanical connection between the first die paddle 111 and the lead frame strip (not shown). The third pin 103 includes a strip inner pin 103 a locating close to and extending along the first transverse edge 111 a of the first die paddle 111 and the first transverse edge 112 a of the second die paddle 112 from a straight line 170 along the first pin 101 and the second pin 102 to a straight line 180 located at the center of and parallel to the first longitudinal edge 112 c and the second longitudinal edge 112 d of the second die paddle 112. The second die paddle 112 includes a connecting rib connected to the first transverse edge 112 a at the corner of the first transverse edge 112 a and the first longitudinal edge 112 d, which is used for a mechanical connection between the second die paddle 112 and the lead frame strip (not show). The fourth pin 104 includes a strip inner pin 104 a locating close to and extending along the second longitudinal edge 112 d of the second die paddle 112.

FIG. 3B is a schematic diagram of the flipped lead frame unit in FIG. 3A. In an embodiment, support pads 1110 and 1120 are respectively attached on the bottom surface of the first die paddle 111 and the second die paddle 112. Moreover, each of the first to fourth pins 101-104 also includes one or more outer pins. The outer pin 101 b of the first pin 101 is in L shape and includes a bottom horizontal part 101″b positioned on the bottom surface of the inner pin 101 a, and a vertical part 101′b positioned on the outer edge of the inner pin 101 a. As shown in FIG. 3B, there are two outer pins are attached to the inner pin 101 a and the bottom horizontal parts 101″b of these two outer pins 101 b is connected together with a connecting part also positioned on the bottom surface of the inner pin 101 a between these two outer pins 101 b. Similarly, the outer pin 102 b of the second pin 102 is in L shape and includes a vertical part 102′b positioned on the outer edge of the inner pin 102 a and a bottom horizontal part 102″b positioned on the bottom surface of the inner pin 102 a. Similarly, the outer pin 103 b of the third pin 103 is in vertical L-shape and includes a vertical part 103′b positioned on the end surface of one end of the third pin 103, which is located on the same straight line 170 along the first pin 101 and the second pin 102 and a bottom horizontal part 103″b locating on an outer area of the bottom surface of the inner pin 103. The inner pin 103 a also includes a rectangular through hole at the middle area of the inner pin 103 a. Similarly, the outer pin 104 b of the fourth pin 104 is in L-shape and includes a vertical part 104′b positioned on the outer edge of the inner pin 104 a and a bottom horizontal part 104″b positioned on the bottom surface of the inner pin 104 a. As shown in FIG. 3B, there are four outer pins 104 b are attached to the inner pin 104 a and the bottom horizontal parts 104″b of the two adjacent outer pins 104 b are connected together with a connecting part positioned on the bottom surface of the inner pin 104 a between these two outer pins 104 b. The bottom surfaces of the bottom horizontal parts 101″b, 102″b, 103″b and 104″b of the outer pins 101 b-104 b and the support pads 1110 and 1120 are coplanar, which is convenient for the subsequent packaging steps.

As shown in FIG. 4A, a first semiconductor chip 121 is attached on the top surface of the first die paddle and a second semiconductor chip 122 is flipped and attached on the inner pin 103 a of the third pin 103 and the second die paddle 112. Both of the first chip 121 and the second chip 122 are vertical power MOSFETs, each includes a first electrode (such as source) and a second electrode (such as gate) formed at the front surface of the chip and a third electrode (such as drain) formed at the back surface of the chip and covered with a metal layer. In some applications, the first chip 121 and the second chip 122 can be bare chips. However, the chip packages formed by the method shown in FIGS. 12A to 12F can be ultra-thin due to the support of the plastic package layer, thus the Rdson of the MOSFET is reduced and a bridge type metal clip can be applied, which will be described in details in the following embodiment. The third electrode of the first chip 121 is attached on the first die paddle 111, while the first electrode at the front of the second chip 122 is attached on the top surface of the second die paddle 112, and the second electrode of the second chip 122 is attached on the inner pin 103 a of the third pin 103 via a conductive adhesive, such as soldering paste and the likes. The first electrode 121 a and the second electrode 121 b of the first chip 121 face upward as shown in FIG. 4A. The second chip 122 is flipped, thus the first and second electrodes facing downward, which are not shown in FIG. 4A.

FIG. 4B is a schematic diagram showing a first interconnecting structure, for example a bridge metal clip, which can cover both of the first chip 121 and the second chip 122. The bridge clip includes a main plate 222, a first side wall 222 a and a second side wall 222 b bent downward and extending along two opposite sides of the main plate 222. Referring to FIG. 7B for another example of a bridge clip, the main plate 222 includes a body part 2222, attached on the first electrode 121 a of the first chip 121 and the third electrode at the back of the second chip 122, and a connecting part 2221. The body part 2222 is substantially rectangular shape with two opposite first and second transverse edges 2222 a and 2222 b and two opposite first and second longitudinal edges 2222 c and 2222 d, and a rectangular notch is provided in the corner of the first longitudinal edge 2222 c and the first transverse edge 2222 a forming an L-shaped structure. As shown in FIG. 7B, the connecting part 2221 transversely extends from the first longitudinal edge 2222 c of the body part 2222 to the first pin 101 with one end of the connecting part 2221 bent downward forming a first side wall 222 a and another end at the second longitudinal edge 2222 d of the body part 2222 also bent downward forming a second side wall 222 b.

The first side wall 222 a of the main plate 222 of the bridge clip is then mounted on the first pin 101 and the second side wall 222 b is then mounted on the fourth pin 104 as shown in FIG. 5A. In one embodiment as shown in FIG. 3A, a longitudinal groove 101 a-1 is formed on the top surface of the inner pin 101 a for plug in the first side wall 222 a of the bridge clip. As shown in FIG. 5A, the first side wall 222 a of the bridge clip is aligned and inserted into the groove 101 a-1 to prevent from sliding. Conductive adhesive (not shown) can be deposited into the groove 101 a-1 to strengthen the mechanical and electrical connection between the first side wall 222 a of the bridge clip and the first pin 101. As shown in FIGS. 7B and 5A, the middle part of the second longitudinal edge 2222 d of the body part 2222 is bent downward to form the second side wall 222 b and the two end parts at the two ends of the second longitudinal edge 2222 d of the body part 2222 respectively close to the first and second transverse edges 2222 a and 2222 b of the body part are bent downward to form two plug pieces 222 b-1 separated from the second side wall 222 b and located at both sides of the second side wall 222 b respectively, where the second side wall 222 b comprises a supporting part 222′b at the bottom of the second side wall 222 b. As shown in FIG. 3A, the inner pin 104 a of the fourth pin 104 includes through holes 104 a-1 and 104 a-2 close to the two ends of the pin 104. As shown in FIG. 5A, the two plug pieces 222 b-1 are correspondingly aligned with and clamped into the two holes 104 a-1 respectively to clamp the bridge metal clip to the fourth pin 104 of the lead frame unit, while the supporting part 222′b is attached on the top surface of the inner pin 104 a of the fourth pin 104 positioned between the two holes 104 a-1 and 104 a-2 via a conductive adhesive. Thus, the first electrode 121 a at the front surface of the first chip 121 and the third electrode on the back surface of the second chip 122 are electrically connected to the first pin 101 and the fourth pin 104 respectively through the bridge clip.

As shown in FIG. 5A, a second interconnecting structure 221, for example a small metal clip in vertical Z shape formed by stamping or punching a metal plate, includes a high horizontal part 221 b and a low horizontal part 221 a, where the high horizontal part 221 b is attached on the second electrode 121 b at the front surface of the first chip 121, while the low horizontal part 221 a is attached on the inner pin 102 a of the second pin 102 through a conductive adhesive. The advantage of the L shape of the body part 2222 is that the high horizontal part 221 b is located in the notch of the body part 2222, as such the device's size is optimized.

FIG. 5B is a cross sectional diagram of the hybrid packaging multi-chip semiconductor device in FIG. 5A along the dotted line AA, where the structure 223 is the cross section of the body part 2222 in FIG. 5A. A groove 224 is formed on the bottom surface of the body part 2222 at an area that is in contact with the third electrode at the back surface of the second chip 122 with at least one of two ends of the groove 224 extending to the periphery of the body part 2222 for releasing the gas accumulated in the conductive adhesive (such as soldering paste) coated on the back surface of the second chip 122 for attaching the body part 2222 on the back surface of the second chip 122, thus reducing the reliability of the electrical performance is avoided. The bridge metal clip can be formed by stamping or punching a metal plate.

FIG. 6 shows an alternative hybrid packaging multi-chip semiconductor device. The differences between the bridge metal clip in FIG. 6 and that of FIG. 5A are as follows: in the bridge metal clip in FIG. 6, the first side wall 222 a of the connecting part 2221 includes a supporting part 222′a horizontally extending from the bottom of the first side wall 222 a to the direction away from first die paddle 111, and the second side wall 222 b includes a supporting part 222′b extending from the bottom of the second side wall 222 b to the direction away from the second die paddle 112. The differences between the lead frame unit used in FIG. 6 and that of FIG. 3A are that the inner pin 101 a of the first pin 101 does not include the groove and the inner pin 104 a of the first pin 104 does not include the through holes. As shown in FIG. 6, the supporting parts 222′a and 222′b are mounted on the top surface of the inner pins 101 a and 104 a of the first pin 101 and fourth pin 105 respectively using a conductive adhesive.

FIGS. 7A to 7B illustrate another alternative hybrid packaging multi-chip semiconductor device. The differences between the lead frame units in FIG. 7A and FIG. 3A are as follows: in the lead frame unit of FIG. 7A, the inner pin 104 a of the fourth pin 104 does not include hole but only includes a long strip groove 104 a-3 formed on the top surface of the inner pin 104 a, where the second side wall 222 b of the bridge clip is aligned with and inserted into the groove 104 a-3 for plug in the bridge clip to the fourth pin 104. A conductive adhesive (not shown) can be deposited in the groove 104 a-3 to strengthen the mechanical and electrical connecting ability between the second side wall 222 b and the fourth pin 104.

FIG. 8 shows another alternative embodiment, in which the small metal clip 221 is replaced by a bonding wire 221′.

FIG. 9 also shows another alternative hybrid packaging multi-chip semiconductor device, in which, compared with the structure of the lead frame unit in FIG. 5A, the structure of the second pin 102′ is similar with that of the first pin 101 and the structure of the first pin 101′ is similar with that of the second pin 102. In addition, compared with the bridge clip shown in FIG. 5A, the body part 2222 of the bridge clip shown in FIG. 9 includes the L shape at the corner of the first longitudinal edge 2222 c and the second transverse edge 2222 b (rather than at the corner of the first longitudinal edge 2222 c and the first transverse edge 2222 a shown in FIG. 5A). As such, the first chip 121 is positioned on the first die paddle 111 differently compared with that shown in FIG. 4A, for example, in FIG. 4A, the first electrode 121 a of the first chip 121 is positioned close to the second transverse edge 111 b of the first die paddle 111 and the second electrode 121 b thereof is close to the first transverse edge 111 a of the first die paddle 111; but in FIG. 9, the first electrode 121 a of the first chip 121 is close to the first transverse edge 111 a of the first die paddle 111 and the second electrode 121 b thereof is close to the second transverse edge 111 b of the first die paddle 111. As shown in FIG. 9, the connecting part 2221 transversely extends from the first longitudinal edge 2222 c of the body part 2222 to the second pin 102′ with one end of the connecting part 2221 bent downward forming the first side wall 222 a that is aligned with and inserted into the groove formed on the top surface of the inner pin of the second pin 102′ to clamp the bridge clip with the second pin 102′. The high horizontal part 221 b of the small clip 221 is thus mounted on the second electrode 121 b of the first chip 121 and the low horizontal part 221 a is mounted on the top surface of the inner pin of the first pin 101′. In FIG. 5A, the connecting part 2221 is located on the position of the first longitudinal edge 2222 c close to the second transverse edge 2222 b; but in FIG. 9, the connecting part 2221 is located on the position of the first longitudinal edge 2222 c close to the first transverse edge 2222 a. The advantage of the L shape of the body part 2222 is that the high horizontal part 221 b of the small clip 221 is located in the notch of the body part 2222, thus the device's size is optimized. Furthermore, the second electrode 121 b is exposed in the notch of the body part 2222. In this embodiment, and the small clip 221 may also be replaced by a bonding wire.

After the first and second chips, the small clip and the bridge clip are mounted on the lead frame unit as described above, a plastic packaging material is deposited to cover the lead frame strip containing a plurality of such lead frame units in the packaging process, and then the lead frame and the plastic packaging material are cut to separate individual hybrid packaging multi-chip semiconductor device. FIGS. 10A and 10B are schematic diagrams from the top surface and from the bottom surface of the hybrid packaging multi-chip semiconductor device after the packaging process. As shown in FIGS. 10A and 10B, the hybrid packaging multi-chip semiconductor device comprises a plastic package body 250 covering the lead frame unit, the first chip 121, the second chip 122, and the first and second interconnecting structures 221 and 222, where the bottom surfaces of the support pads 1110 and 1120 at the respective bottom of the first die paddle 111 and the second die paddle 112, the bottom surfaces of the bottom horizontal parts 101″b-104″b of the respective outer pins 101 b-104 b of the first to fourth pins 101-104 and the connecting part connecting the bottom horizontal part 104″b of two adjacent outer pins 104 b of the fourth pin 104 are exposed out of the bottom surface of the plastic package body 250, and the top surface of the L-shaped structure 223 is exposed out of the top surface of the plastic package body 250.

For the bridge clip type used in the hybrid packaging multi-chip semiconductor device structure shown in FIG. 7B, the main plate 222 is not really flat as the structure 223 of the FIG. 5A since the bridge clip of FIG. 7B is directly formed by mechanically stamping or punching; thus, the main plate 222 of the bridge clip of FIG. 7B may be completely covered inside of the plastic package body 250. In another embodiment as shown in FIGS. 11A and 11B, which are the schematic diagrams of the top surface and the bottom surface of the hybrid packaging multi-chip semiconductor device, the bottom surfaces of the support pads 1110 and 1120 at the respective bottom of the first die paddle 111 and the second die paddle 112, the bottom surfaces of the bottom horizontal parts 101″b-104″b of the respective outer pins 101 b-104 b of the first to fourth pins 101-104, and the connecting part connecting the bottom horizontal part 104″b of all four adjacent outer pins 104 b of the fourth pin 104 are exposed out of the bottom surface of the plastic package body 250, while the plastic package body 250 completely covers the top of the hybrid packaging multi-chip semiconductor device.

FIGS. 12A to 12F are cross sectional schematic diagrams illustrating a wafer-level method for preparing the first semiconductor chip 121 and the second semiconductor chip 122 on a semiconductor wafer 300. Firstly, a metal pump 305 (such as solder ball or solder paste) is deposited on the electrode pad 301, such as the source electrode pad or gate electrode pad, and then the wafer 300 is cut along a scribe line 302 (see FIG. 13A) at the front surface forming a scribe groove 315 at a top portion of the wafer 300 as shown in FIG. 12B. Then, a plastic package material is deposited to cover the front surface of the wafer 300 forming the plastic package layer 306 covering the metal bump 305 and also filling in the scribe groove 315, as shown in FIG. 12C. The plastic package layer 306 does not cover a ring area 303 at the edge of the front surface of the wafer 300, as shown in FIG. 13B, as such both ends of each scribe line 302 are exposed in the ring area 303. The plastic package layer 306 is ground from the top surface to expose the metal bumps 305, as shown in FIG. 12D. Thus, the plastic package materials filled in each scribe groove 315 are cut through a straight line determined by two ends of a scribe line exposed in the ring area 303 forming a plurality of longitudinal and transverse reference lines 325 on the plastic package layer 306, as shown in FIGS. 12E and 13B, where each reference line 325 is overlapped with the corresponding scribe groove 315 as shown in FIG. 13C. And then, the wafer 300 is ground from the back surface, for example with CMP method. The thin wafer 300 is then covered with a metal layer on the back surface followed by the cutting along the reference line 325 to separate a plurality of chips 300′, each of which includes a top plastic package layer 306′ formed by cutting the plastic package layer 306 covering the front surface of the chip 300′ and a back metal 307′ formed by cutting the metal layer covering the back surface of the chip 300′. As shown in FIG. 12F, the top plastic package layer 306′ further comprises a side plastic package layer 306′a surrounding the top portion of semiconductor sidewalls of the chip 300′ adjacent to the front surface of the chip. The bottom portion of semiconductor sidewalls of chip 300′ adjacent to the bottom surface of the chip is not covered by the side plastic package layer 306′a and is coplanar to an outside surface of the side plastic package layer 306′a. If the metal bump 305 is an adhesive, for example a soldering material, the chip is attached on the die paddle or the metal clip directly by the metal bumps. However, if the metal bump 305 is not an adhesive, for example a metal such as copper, an adhesive is used for attaching the chip on the die paddle or the metal clip.

In an embodiment, the first chip 121 is a high-side MOSFET, the metal bump 305 exposed from the top plastic package layer 306′ and located on the first electrode is used for attaching to the body part 2222 of the bridge clip, the metal bump 305 exposed from the top plastic package layer 306′ and located on the second electrode is used for attaching the small clip 221, while the third electrode 307′ is directly attached to the top surface of the first die paddle 111 through a conductive adhesive. Furthermore, the second chip 122 is a low-side MOSFET, the metal bump 305 exposed from the top plastic package layer 306′ and located on the first electrode is used for attaching on the second die paddle 112, the metal bump 305 exposed from the top plastic package layer 306′ and located on the second electrode is used for attaching to the inner pin 103 a, while the third electrode 307′ is directly attached to the body part 2222 of the bridge clip through a conductive adhesive. As shown in FIGS. 10A and 11A, the plastic package body 250 also covers the top plastic package layer 306′ of the first and second chips 121 and 122.

The above detailed descriptions are provided to illustrate specific embodiments of the present invention and are not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is defined by the appended claims. 

The invention claimed is:
 1. A preparation method of a hybrid packaging multi-chip semiconductor device, the preparation method comprising: providing a lead frame unit, the lead frame unit comprising: a first die paddle and a second die paddle disposed side by side with a gap therebetween separating the first die paddle and a second die paddle; a first pin and a second pin disposed along a first side of the first die paddle away from the second die paddle; and a third pin disposed along a first transverse edge of the first die paddle and a second transverse edge of the second die paddle; attaching a first semiconductor chip onto a top surface of the first die paddle and attaching a second semiconductor chip on the third pin and on the second die paddle, wherein a first electrode and a second electrode at a front surface of the second semiconductor chip are electrically connected to the second die paddle and the third pin respectively; and attaching a first interconnecting structure on the first semiconductor chip and the second semiconductor chip, wherein the first interconnecting structure comprises a main plate having at least a first side wall bending downward at a side of the main plate; wherein the first side wall is mounted on a selected pin of the first pin and the second pin, so that a first electrode at a front surface of the first semiconductor chip and a third electrode at a back surface of the second semiconductor chip are electrically connected to the selected pin of the first pin and the second pin; and wherein a second electrode at the front surface of the first semiconductor chip is electrically connected to an other pin of the first pin and the second pin through a second interconnecting structure.
 2. The preparation method according to claim 1, wherein the main plate comprises a body part having a notch at a corner, the body part forming an L-shaped structure; and a connecting part transversely extending from the body part to the first pin or the second pin, one end of the connecting part being bent downward to form the first side wall; wherein the body part is mounted on the first semiconductor chip and the second semiconductor chip and the body part electrically connects the first electrode at the front surface of the first semiconductor chip to the third electrode at the back surface of the second semiconductor chip; and wherein the second electrode at the front surface of the first semiconductor chip is exposed from the notch.
 3. The preparation method according to claim 2, wherein the second interconnecting structure comprises a vertical Z-shaped metal clip including a high horizontal part and a low horizontal part, and wherein the high horizontal part is attached to the second electrode of the first semiconductor chip and the low horizontal part is attached to the other pin of the first pin or the second pin.
 4. The preparation method according to claim 2, wherein a groove is formed on a bottom surface of the body part at an area near the third electrode at the back surface of the second semiconductor chip, wherein end portions of the groove extend to a periphery of the body part; wherein the body part is mounted on the third electrode at the back surface of the second semiconductor chip by a conductive adhesive and wherein the groove is used for releasing gas accumulated in the conductive adhesive.
 5. The preparation method according to claim 1, wherein the second interconnecting structure comprises a bonding wire with one end bonded on the second electrode at the front surface of the first semiconductor chip and an other end bonded on either one of the first pin or the second pin not connecting to the first side wall.
 6. The preparation method according to claim 1, wherein a longitudinal strip groove is formed on a top surface of the selected pin of the first pin and the second pin for attaching the first side wall; and wherein the first side wall is aligned with and inserted into the longitudinal strip groove to lock the first interconnecting structure.
 7. The preparation method according to claim 1, wherein the first side wall comprises a supporting part mounted on a top surface of either one of the first pin or the second pin.
 8. The preparation method according to claim 1, wherein the lead frame unit further comprises a fourth pin disposed along a longitudinal edge of the second die paddle away from the first die paddle, wherein the first interconnecting structure further comprises a second side wall bent downward and located at a side of the main plate opposite the first side wall, and wherein the second side wall is electrically connected to the fourth pin.
 9. The preparation method according to claim 8, wherein a longitudinal strip groove is formed on a top surface of the fourth pin; and wherein the second side wall is aligned with and inserted into the longitudinal strip groove to clamp the first interconnecting structure.
 10. The preparation method according to claim 1, wherein the lead frame unit further comprises a fourth pin disposed along a longitudinal edge of the second die paddle away from the first die paddle, and wherein a middle part located at a center of an edge of the main plate opposite the first side wall is bent downward to form a second side wall contacting the fourth pin.
 11. The preparation method according to claim 10, wherein two end parts away from the middle part located at the edge of the main plate are bent downward to form two clamping pieces, wherein the two clamping pieces are separated from the second side wall and positioned at both sides of the second side wall; wherein two clamping holes are formed in the fourth pin, wherein the second side wall comprises a supporting part; wherein the two clamping pieces are correspondingly clamped into the two clamping holes and wherein the supporting part is attached to a top surface of the fourth pin between the two clamping holes.
 12. The preparation method according to claim 1, wherein bottom surfaces of the first die paddle and the second die paddle are attached with support pads; wherein each of the first pin, the second pin and the third pin comprises a bottom horizontal part, and wherein a respective bottom surface of the bottom horizontal part is coplanar with bottom surfaces of the support pads of the first die paddle and the second die paddle.
 13. The preparation method according to claim 12, wherein the main plate comprises a body part having a notch at a corner, the body part forming an L-shaped structure.
 14. The preparation method according to claim 12 further comprising a step of depositing a plastic package body for covering the lead frame unit, the first semiconductor chip, the second semiconductor chip, the first interconnecting structure and the second interconnecting structure, wherein the bottom surfaces of the support pads of the first die paddle and the second die paddle and the bottom surfaces of the bottom horizontal parts of the first pin, the second pin and the third pin are exposed from a bottom surface of the plastic package body.
 15. The preparation method according to claim 14, wherein a top surface of the main plate of the first interconnecting structure is exposed from a top surface of the plastic package body.
 16. The preparation method according to claim 1, wherein the first die paddle further comprises a connecting rib extending between the first pin and the second pin. 